1. Field of the Invention
The present invention relates to a folded cascode differential amplifier having both high-voltage and low-voltage transistors, and to a semiconductor device including the amplifier and its bias circuit.
2. Description of the Related Art
Differential amplifiers used in liquid crystal displays, for example, have inputs that may vary over a wide common-mode voltage range, even though the differential input voltage range is comparatively small and the output voltage range may be even smaller. A conventional differential amplifier of this type employs two folded cascode differential amplifiers, using one as an input stage and the other as an output stage. The input amplifier has high-voltage transistors and operates on a relatively high power supply voltage. The output amplifier has low-voltage transistors and operates on a relatively low power supply voltage.
Since each of the two folded cascode differential amplifiers has a large number of transistors, one problem with this conventional two-stage design is that it takes up considerable space. A further problem is large output error, due to inaccurate matching of resistance ratios and to offset inaccuracies. Further details will be given in the detailed description of the invention.
Other examples of amplifiers used in liquid crystal displays can be found in, for example, Japanese Patent Application Publications No. 2009-070211, 2007-148428, and 2005-025596.